The current trend in microelectronic packaging is toward high performance applications which demand both a higher density of electronic functionality and functionalities which operate at higher speeds. Producing electronic packages which operate reliably and meet these demands of high performance applications is a challenge facing the microelectronic industry. One particularly difficult aspect of producing such packages is producing joints or bonds in multilayer packages between metallized components such as pins, leads, window frames or heat sinks and ceramic substrates. These joints or bonds must have the mechanical durability, electrical conductivity and heat dissipation necessary to ensure reliable performance under the varying and extreme conditions encountered in high performance applications. Further, the process or method of joining the metallized components to the ceramic substrate must be as cost effective and simple as possible.
Currently in multilayer electronic packaging, there are several methods for attaching metallized components to ceramic substrates. The attachment method used is dependent upon the type of multilayer package being fabricated. For high temperature cofired systems which use alumina-based dielectric layers and tungsten or molybdenum metallizations, a brazing method is used for attachment. The brazing is carried out at a temperature of about 840.degree. C. in a hydrogen-nitrogen atmosphere. This method results in good bond strengths and allows for temperature latitude in subsequent processing.
Low temperature thick film or cofired dielectric sheet systems use glass and alumina-based diectrics and gold, silver or copper based metallization. Brazing has not been successful with low-temperature systems because the temperatures of firing and brazing are essentially the same. This creates a condition where, during the brazing operation, the braze attacks the previously formed bond between the metallization and the ceramic, causing the metallization to separate from the substrate. A disruption of the electrical conductivity results rendering the package useless. Common methods of attachment which are used in lieu of brazing for these low temperature systems include soldering, wirebonding and welding, such as thermal compression and parallel gap welding.
Both of the foregoing systems including the respective methods of attachment are currently in use. However, these systems suffer from some disadvantages in high performance applications. In the high-temperature cofired packages, the electrical conductivity of the metallizations formed with tungsten and molybdenum is not as high as desired for high performance applications. In the low-temperature packages, the conductivity of the gold, silver or copper metallization is good, but the bond strengths are typically lower than those obtained by brazing and are not as high as desired for high performance applications. In addition, solder joint integrity can be lost during some subsequent processing operations.
The effect of subsequent operations on prior process steps is an important consideration in electronic package manufacture. For example, the process of attaching pins using brazing methods should not affect the integrity of the sintered circuitry in the package. Similarly, the subsequent process of attaching a semiconductor chip should not affect the integrity of the brazed pins or sintered circuit lines. If the temperature of a subsequent step is as high or higher than the temperature of a previous step, the integrity of the product from the previous step will very likely be affected by the subsequent step. To avoid such complications, electronic package manufacturers strive to design processes with a temperature hierarchy. Then, the integrity of each process step is preserved throughout the entire manufacturing process. In fact, the higher the temperature at which the last process step occurs in the formation of a complete package, the greater the temperature interval between subsequent processing and the more likely the package will be unaffected by thermal influences. Another advantage of a process having a temperature hierarchy is that the process can become modular. Thus, one can test subunits before final assembly, thereby easing diagnostic work and assuring high final quality levels in the system product.
Therefore, it is an object of this invention to provide a process of brazing metallized components to ceramic substrates in such a way as to form a strong joint and not cause a loss of adhesion at the interface of the metallization and the ceramic substrate. It is also an object of this invention to provide a process of brazing metallized components to ceramic, glassceramic and metal substrates using metal paste that has high electrical conductivity. And, it is a further object of this invention to provide a reliable method for high temperature attachment of metallized components to a ceramic based substrate while preserving the temperature hierarchy of the low temperature fired or cofired systems and allowing the same subsequent processing methods commonly used in high temperature cofired systems.